Image pickup tube with screen and field grids

ABSTRACT

The acceleration and focusing means located between a photocathode and a secondary emission induced conductivity target include a grid located and electrically energized to reduce the speed of electrons impinging on the target to a value providing for optimum secondary emission from the target, thus assuring low electron transit times and high resolution.

United States Patent Inventor Lucien Francis Guyot Paris, France Appl.No. 759,669 Filed Sept; 13, 1968 Patented Feb. 9, 1971 AssigneeCompagnie Francaise Thomson-Hotchkiss Brandt Paris, France a corporationof France Priority Sept. 28, 1967 France IMAGE PICK-UP TUBE WITH SCREENAND FIELD GRIDS 1 Claim, 3 Drawing Figs.

US. Cl 313/65, 315/1 1 Int. Cl ..H0lj 31/28, H01 j 3 1/38 Field ofSearch 313/65 [56] References Cited UNITED STATES PATENTS 2,452,61911/1948 Weimer 313/65 2,460,093 1/1949 Law 313/65 2,723,360 11/1955Rotow 313/65 2,871,368 1/1959 Bain 313/65X 2,983,836 5/1961 Rudnick etal.... 3 l3/65X 3,303,373 2/1967 Alting-Mees 313/65 FOREIGN PATENTS1,137,910 12/1968 Great Britain. 1,276,084 8/1968 Germany.

Primary Examiner-Robert Sega] Attorney-Stephen H. Frishauf ABSTRACT: Theacceleration and focusing means located between a photocathode and asecondary emission induced conductivity target include a grid locatedand electrically energized to reduce the speed of electrons impinging onthe target to a value providing for optimum secondary emission from thetarget, thus assuring low electron transit times and high resolution.

PATENTEUFEB sum 35 21515 sum 2 UF 2 IMAGE PICK-UP TUBE WITH SCREEN ANDFIELD GRllDS The present invention relates to image pickup tubes, andmore particularly to tubes having targets of the secondary emissioninduced conductivity type, which are read out by means of a slowelectron beam, and which have high resolution.

Image pickup tubes of the type with which the present invention isconcerned, usually are arranged, in the order of the longitudinal axisof the tube, to have first an emissive cathode, emitting electrons as afunction of the local density of impinging electromagnetic orcorpuscular radiation, and forming the primary image to be scanned. Ifthe radiation is not of the type which provides for direct excitation ofthe cathode, an inter mediate luminescent image converter may beprovided, for example by means of a fluorescent screen joined to thecathode. Further, the tube includes acceleration and focusing means,usually forming electrostatic lenses, in order to focus the electrons ona target. The target, of the secondary emission induced conductivitytype, is usually formed by an insulating material having a highsecondary emission coefficient, located on a metallic membrane which ispermeable to highly accelerated electrons. The membrane is oriented tobe directed towards the cathode. It has a high potential. with respectto the cathode. It has a high potential, with respect to the cathode,connected thereto, usually in the range of from to kilovolts. Themembrane, itself, may be supported at the side facing the cathode, by aninsulating member which is also permeable to fast electrons.

in addition, the readout tube comprises an electron gun providing ascanning electron beam, directed towards the porous layer of the target,and scanning thereover.

in operation, the electrons emitted from the cathode cross the metallicmembrance of the target and liberate, within the porous layer, a largenumber of secondary electrons which are directed towards the membrane,leaving the porous layer with a charge image, thus fomiing an electricalimage corresponding to the primary image reaching the photocathode. Thescanning electron beam, formed of slow electrons, restores the potentialand charge gradients formed on the target, thus causing within thesupply conductor to the membrane the output signal current.

Tubes of the type described function satisfactorily so long as theacceleration and focusing system is comparatively short, for example afew centimeters. The diameter of the cathode is thus limited to valuesof the same order of magnitude. It is desirable, however, to increasethe diameter of the cathode in order to improve resolution andsensitivity of the tube, or to increase its field of vision, a featurewhich is particularly important in X-ray luminescent amplifiers. It hasbeen found, however, when the diameter of the cathode is increased, thusnecessitating increase of the overall dimensions of the acceleration andfocusing system, the resolution of the tube deteriorates. it is believedthat this is due to the fact that the electrons coming from one spot ofthe cathode produce as an image at that point, a spot on the targetwhich is larger than the spot from which they were derived. Thisphenomenon appears to be due to the fact that the initial speed of theelectron is distributed at random in all spatial directions, so thatradial components of speed will be present, thus increasing elementalareas of electron bundles directed towards the target, during thetransit time of the electrons. The increase in transit time of theelectrons, as a result of the increase in distances, may theoretically,be compensated by increase in the accelerating potential. Inelectrostatic systems, however, even using magnetic focusing, thetransit time varies as the inverse of the square root of potential.Thus, in a system operating, for example, at an ordinary potential of 7kv., doubling of the geometric dimensions requires an increase involtage to 28 kv. in order to retain the same transit time for theelectrons. lf magnetic focusing is then used, the weight and auxiliaryequipment of the entire tube assembly will become prohibitive.

In addition, increase of the accelerating potential has as a result adecrease in the efficicncy of the target, which may be so great as to beinadmissible in actual practice. ln effect. the materials which can beused for the porous layer of the target have a maximum net coefficientof secondary emission with respect to incident energies of electronswhich are much less than l0 Kev.

It is an object of the present invention to provide an image pickup tubeof the type described which permits use ofa large area photocathodewithout requiring heavy auxiliary equipment or focusing structures andproviding images of high resolution.

Subject matter of the present invention: Briefly, the acceleration andfocusing arrangement of the tubes include, as the last electrode, agrid, hereinafter termed a field grid, having a transparency of at least60 percent, and presenting a structure which is sufficiently fine and solocated that it is at a distance sufficiently small from the target sothat when a dif' ference of potential between the grid and target isapplied. an essentially homogeneous field is produced between target andgrid.

In operation of the tube, the accelerating potential of a value desiredfor the accelerating and focusing system is applied to the grid, whereasthe target is carried with respect to the cathode at a potential chosento provide optimum secondary emission from the porous layer.

in accordance with an embodiment of the invention, an additional grid,termed a screen grid, can be placed between the first mentioned, fieldgrid and the target, and spaced closely from the target. it is connectedto a potential which is similar to that of the target, or close thereto;it protects the target from the electrical field resulting from thedifference in potential between grid and target itself.

The structure, organization, and operation of the invention will now bedescribed more specifically with reference to the accompanying drawings,wherein:

FIG. 1 is a longitudinal cross section, somewhat schematic view of anentire tube in accordance with the present invention;

FIG. 2 is a greatly enlarged schematic view of the field gridtargetassembly of the tube of HO. 1; and

FIG. 3 is a different embodiment of the assembly of F K}. 2.

The tube generally designated with I has an image pickup section 2 and areadout section 3, the entire assembly being placed in an evacuatedenvelope 4.

The image pickup area forming face 5 of the envelope is connected to theremainder of the tube, which may be of glass, by a pair of annularmetallic seals, 6,7, to support a photocathode 8. Photocathode 8 emitsphotoelectrons having a localized density corresponding to localizedbrightness of an image projected thereonto. An assembly of electrodes9,10,11, accelerates and focuses the image on a secondary emissioninduced conductivity type target 12. Target 12 is supported by a ceramiccollar 13. Collar 13 is, itself, maintained in position within theenvelope 4 by means of a group of support stems, of which two, 14 and15, can be seen on FIG. 1. Target 12, and the far end of electrode 11adjacent thereto are shown in greater detail in FIG. 2, where therepresentation is schematic and without considering the relative sizesand proportions of the various elements. As an example, target 12 may beformed by a support membrane 16 made of alumina of a thickness of from0.05 to 0.1 microns, an aluminum layer 17 of, for example, 0.03 micronsin thickness, and a layer 18 of potassium chloride which is very porous,and having a thickness of from 15 to 20 microns. Layer 17 is the signalelectrode of the tube. its supply conductor is one of the support stems,for example stem 14 as shown.

Electrode 11 carries, at its side closest to the target, a diaphragm theopening of which is covered by a mesh grid 19. Mesh grid 19 is formed ofwires 20, of copper or nickel, and very fine, for example having adiameter of from 5 to 8 microns with a spacing p of from 25 to 50microns to provide a mesh of from 20 to 40 mesh per millimeter. Thetransparency of such a grid will be greater than 60 percent. Grid 19,which forms the field grid of the system is located from target 12 by adistance d of from 5 to l millimeters.

As seen in FIG. 1, cathode 8, to which the first electrode 9 isconnected, electrodes 10 and l l as well as electrode 17 of the target12 are connected to tap points of a potentiometer 21, supplied by asource of potential 22. A resistance 23, inserted in the supply linefrom electrode 17 to its connection to the potentiometer 21, provides adropping resistor for a signal output, taken off lead 24. Electrodes 10,11 and signal electrode 17 are placed, for example, with respect to thepotential of the cathode 8, at voltages of +2.5 kv +25 kv, and +10 kv.

Section 3 of the tube contains an electron gun, formed of athermoemissive cathode 25, followed by a control electrode 26 andpositively connected electrodes 27, 28. Electrode 28 extends in form ofa hollow cylinder and terminates at a fine mesh 29 forming a grid, andlocated close to the target 12. A magnetic coil 30, located around theexterior of envelope 4 provides a magnetic field in an axial direction,cooperating with the electrostatic lens elements formed by electrodes26,27,28 so that the electrons emitted from the cathode 25 form a verynarrow, fine pencil beam which is directed almost perpendicularly ontothe target 12. A coil 31 is arranged to provide for alignment of theelectron beam with the axis of the assembly. The electrodes areconnected to a potentiometer 32, which is supplied by a source 33. Anintermediate point 39 on potentiometer 32 is connected to a junction 34which also forms the other terminal of resistance 23, further connectedto ground, across which the output potential appears. The ground pointinterconnects the supply of the input section 2 and the readout section3. For example, cathode 25 may be carried at a potential of l0 v withrespect to the target, and electrodes 26, 27, 28 may have, respectively,potentials of 60 v, +270 v, and +300 v with respect to cathode 25. Twopairs of deflection coils, of which 35, 36 only are seen on the drawingprovide for scanning and deflection of the electron beam readout acrosstarget 12.

The electrons emitted by the photocathode are thus accelerated into theregion of the target by a very intense electric field. A powerfulacceleration system may be used, and thus the diameter of thephotocathode may be substantial, for example in excess of 20 cm. withoutdeterioration of quality of the secondary image on the target due totransit time effects. The electrons, after having received an energy offrom 20 to 30 Kev. are, however, slowed upon passing through the fieldgrid 19 and reach the target 12 with an energy appropriate to providemaximum secondary emission therefrom, that is, with an energy of fromabout to Kev. The target may thus present the usual, customary diameterof several centimeters only, the size of which is determined by itsfragile structure. The deceleration field between grid and target issubstantial, and practically homogeneous, due to the fine structure ofthe mesh grid and the small distance form the target itself. Eachelementary bundle of electrons emitted from any one point of thephotocathode is thus focused.

Little experimentation or adjustment is necessary in order to determinethe optimum relative values of distance and potential differencesbetween the mesh grid and the target so that a sharp image of highresolution is projected on the target. it also appears that the strongdecelerating field between grid 19 and target 12 improves the emissionof secondary electrons from the face of target 12 impinged by theelectrons.

Referring now to H6. 3, an additional grid 37 which may be termed ascreen grid may be located between the field grid 19 and target 12. Grid37 is supported by a metallic collar or ring, located in a support ring13 of the target and spaced by a distance d. of from about 0.2 to 1 mm.from the target. The current supply conductor may be one of the supportstems, for example, stem 15. Just like the field grid 19, screen grid 37may be a mesh grid made of fine wires from 5 to 8 microns in diameter,but located at a wider distance apart, for example between 50 to micronsso that a mesh of from l2 to 20 mesh per millimeter will be formed,resulting in a greater transparency, preferably in excess of 80 percent.It is not necessary that the electric field between grid 37 and target12 is as even and homogeneous as that caused by the field grid, sincethe screen grid is connected to a potential which is close to that ofthe target, or even the same; the trajectory of the electrodes whichpass the grid thus are not substantially modified.

Screen grid 37 places target 12 into the shadow of an electrostaticfield if there is a potential difference therebetween. lntroduction ofthe screen grid is advantageous if the field between field grid 19 andtarget is high, and particularly if the field has a physical effect onthe target. which may eventually cause an internal short circuit orarc-over or a mechanical deformation of the target which may cause itsdestruction, particularly when, during use of the tube, shocks ormechanical vibrations may be expected. The screen grid further collectssecondary electrons emitted from the target surface sub ject toimpingement by the electrons from the photocathode. Additionally, thescreen grid permits construction of a thinner target structure-as seenin FIG. 3in which target 12 merely consists of a signal pickup plate 17fonned of an aluminum membrane of 0.07 micron thickness, which supportsthe porous layer 18 of potassium chloride.

Placing a fine mesh grid thus permits manufacture of tubes having largediameter while maintaining high resolution of readout.

The invention has been described particularly with respect to anelectron tube in which a cathode emits electrons as a function oflocalized brightness and density patterns of electromagnetic, orcorpuscular radiation impinging on the target face. It is, however,understood that the present invention is equally applicable to tubesresponsive to other types of radiation, and that various changes andmodifications may be made in the structure and arrangement of the tubein accordance with specific uses, within the inventive concept.

I claim:

1. An image pickup tube comprising a photoelectric cathode (8) emittingelectrons as a function of impinging radiation in accordance with theimage;

a porous storage target (12) of the secondary emission inducedconductivity type;

means scanning reading electrons over said target;

means to accelerate and focus the electrons including:

a fine mesh field grid (19) having a transparency of at least Q0 percentand closely spaced from said target (12), said field grid beingconnected to a source of potential of such magnitude relative to thespacing of the grid that the electrical field between grid (19) andtarget (12) is substantially uniform and a screen grid (FIG. 3:37)having a transparency of more than 80 percent located between the target(12) and said first grid (19), said screen grid being located close tosaid target, said field grid (19) being formed of wires of from 5 to 8microns diameter, having a mesh spacing of from 25 to 30 mesh permillimeter and located from 5 to 10 millimeters from the target; saidscreen grid being located from the target by a distance of from 0.2 to lmillimeter, and formed of wire of from 5 to 8 microns diameter, having amesh spacing of from 12 to 20 mesh per millimeter.

1. An image pickup tube comprising a photoelectric cathode (8) emittingelectrons as a function of impinging radiation in accordance with theimage; a porous storage target (12) of the secondary emission inducedconductivity type; means scanning reading electrons over said target;means to accelerate and focus the electrons including: a fine mesh fieldgrid (19) having a transparency of at least 60 percent and closelyspaced from said target (12), said field grid being connected to asource of potential of such magnitude relative to the spacing of thegrid that the electrical field between grid (19) and target (12) issubstantially uniform and a screen grid (FIG. 3:37) having atransparency of more than 80 percent located between the target (12) andsaid first grid (19), said screen grid being located close to saidtarget, said field grid (19) being formed of wires of from 5 to 8microns diameter, having a mesh spacing of from 25 to 30 mesh permillimeter and located from 5 to 10 millimeters from the target; saidscreen grid being located from the target by a distance of from 0.2 to 1millimeter, and formed of wire of from 5 to 8 microns diameter, having amesh spacing of from 12 to 20 mesh per millimeter.